Circulating fluid bed (CFB) reactors are well known devices that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material at velocities high enough to suspend the solid and cause it to behave as though it were a fluid. Fluidization is maintained by means of fluidizing gas such as air, steam or reactant gas injected through a distributor (grid, spargers or other means) at the base of the reactor. CFB reactors are now used in many industrial applications, among which are catalytic cracking of petroleum heavy oils, olefin polymerization, coal gasification, and water and waste treatment. One major utility is in the field of circulating fluid bed combustors where coal or another high sulfur fuel is burned in the presence of limestone to reduce SOx emissions; emissions of nitrogen oxides is also reduced as a result of the relatively lower temperatures attained in the bed. Another application is in the fluidized bed coking processes known as fluid coking and its variant, Flexicoking™, both of which were developed by Exxon Research and Engineering Company.
Fluidized bed coking is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residue (resid) from fractionation or heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 480 to 590° C., (about 900 to 1100° F.) and in most cases from 500 to 550° C. (about 930 to 1020° F.). Heavy oils which may be processed by the fluid coking process include heavy atmospheric resids, aromatic extracts, asphalts, and bitumens from oil sands, tar pits and pitch lakes of Canada (Athabasca, Alta.), Trinidad, Southern California (La Brea, Los Angeles), McKittrick (Bakersfield, Calif.), Carpinteria (Santa Barbara County, Calif.), Lake Bermudez (Venezuela) and similar deposits such as those found in Texas, Peru, Iran, Russia and Poland. The process is carried out in a unit with a large reactor vessel containing hot coke particles which are maintained in the fluidized condition at the required reaction temperature with steam injected at the bottom of the vessel with the average direction of movement of the coke particles being downwards through the bed. The heavy oil feed is heated to a pumpable temperature, typically in the range of 350 to 400° C. (about 660 to 750° F.) mixed with atomizing steam, and fed through multiple feed nozzles arranged at several successive levels in the reactor. Steam assisted atomization nozzles are used to spray the heavy oil feed into a fluidized bed of hot coke particles. The injected spray forms a jet in the bed into which fluidized coke particles are entrained. Effective mixing of the atomized feed droplets and the entrained coke particles is vital for improving reactor operability and liquid yield.
Fluidization steam is injected into a stripper section at the bottom of the reactor and passes upwards through the coke particles in the stripper as they descend from the main part of the reactor above. A part of the feed liquid coats the coke particles in the fluidized bed and subsequently decomposes into layers of solid coke and lighter products which evolve as gas or vaporized liquid. Reactor pressure is relatively low in order to favor vaporization of the hydrocarbon vapors, typically in the range of about 120 to 400 kPag (about 17 to 58 psig), and most usually from about 200 to 350 kPag (about 29 to 51 psig). The light hydrocarbon products of the coking (thermal cracking) reactions vaporize, mix with the fluidizing steam and pass upwardly through the fluidized bed into a dilute phase zone above the dense fluidized bed of coke particles. This mixture of vaporized hydrocarbon products formed in the coking reactions continues to flow upwardly through the dilute phase with the steam at superficial velocities of about 1 to 2 meters per second (about 3 to 6 feet per second), entraining some fine solid particles of coke. Most of the entrained solids are separated from the gas phase by centrifugal force in one or more cyclone separators, and are returned to the dense fluidized bed by gravity through the cyclone diplegs. The mixture of steam and hydrocarbon vapor from the reactor is subsequently discharged from the cyclone gas outlets into a scrubber section in a plenum located above the reaction section and separated from it by a partition. It is quenched in the scrubber section by contact with liquid descending over scrubber sheds in a scrubber section. A pumparound loop circulates condensed liquid to an external cooler and back to the top row of scrubber section to provide cooling for the quench and condensation of the heaviest fraction of the liquid product. This heavy fraction is typically recycled to extinction by feeding back to the fluidized bed reaction zone.
The Flexicoking™ process, also developed by Exxon Research and Engineering Company, is, in fact, a fluid coking process that is operated in a unit including a reactor and burner, often referred to as a heater in this variant of the process, as described above but also including a gasifier for gasifying the coke product by reaction with an air/steam mixture to form a low heating value fuel gas. The heater, in this case, is operated with an oxygen depleted environment. The gasifier product gas, containing entrained coke particles, is returned to the heater to provide a portion of the reactor heat requirement. A return stream of coke sent from the gasifier to the heater provides the remainder of the heat requirement. Hot coke gas leaving the heater is used to generate high-pressure steam before being processed for cleanup. The coke product is continuously removed from the reactor. In view of the similarity between the Flexicoking process and the fluid coking process, the term “fluid coking” is used in this specification to refer to and comprehend both fluid coking and Flexicoking except when a differentiation is required.
The dense fluid bed behaves generally as a well-mixed reactor. However model simulations using cold flow dynamics and tracer studies have shown that significant amounts of wetted coke can rapidly bypass the reaction section and contact the stripper sheds where a portion of the wet film is converted to coke, binding the coke particles together. Over time, hydrocarbon species from the vapor phase condense in the interstices between the particles, creating deposits which are very hard and difficult to remove.
One approach both to reducing reactor fouling and to increase liquid yield has been to improve the atomization of the feed as it enters the bed with the expectation that improved atomization will reduce the extent to which the oil will be carried down in liquid form into the stripper. Conventional atomization nozzles used in the fluid coking process use steam to assist in spraying the heated resid or bitumen into the fluidized bed of hot coke particles: effective contacting of resid droplets and the entrained coke particles is important in improving reactor operability and liquid product yield. The injected spray forms a jet in the bed into which fluidized coke particles are entrained. A major concern with poorly performing atomization nozzles is that liquid-solid agglomerates tend to form in the bed, causing high local liquid loading on the solids with the formation of large wet feed/coke agglomerates with particle sizes substantially larger than the bulk solids average. These heavier agglomerates may tend to segregate towards the lower section of the reactor and foul the internals of the reactor, particularly in the stripper section. These agglomerates also suffer from increased heat and mass transfer limitations and reduce liquid yields. With enhanced feed atomization performance, the contacting between the atomized feed and coke solids would be improved, resulting in an overall improvement in reactor operability, with longer run-lengths due to reduced reactor fouling, and/or higher liquid product yield due to lower reactor temperature operation. By spreading the liquid more evenly over the coke particles thinner liquid films would be created, reducing the heat and mass transfer limitations with liquid yields. Higher liquid feed rates may also be facilitated by the use of improved feed nozzles.
A steam assisted nozzle proposed for use in fluid coking units is described in U.S. Pat. No. 6,003,789 (Base) and CA 2 224 615 (Chan). In this nozzle, which is typically mounted on the side wall of the fluid coker so that it extends through the wall into the fluidized bed of coke particles, a bubbly flow stream of a heavy oil/steam mixture is produced and atomized at the nozzle orifice. The nozzle which is used has a circular flow passageway comprising in sequence: an inlet; a first convergence or contraction section of reducing diameter; a diffuser section of expanding diameter; a second contraction section of reducing diameter; and an orifice outlet. The convergent sections accelerate the flow mixture and induce bubble size reduction by elongation and shear stress flow mechanisms. The second contraction section is designed to accelerate the mixture flow more than the first contraction section and as a result, the bubbles produced by the first contraction are further reduced in size in the second contraction. The diffuser section allows the mixture to decelerate and slow down before being accelerated for the second time. The objective is to reduce the average mean diameter of the droplets exiting the nozzle to a relatively fine size, typically in the order of 300 μm as it is reported that the highest probability of collision of heavy oil droplets with heated coke particles occurs when both the droplets and heated particles have similar diameters; thus a droplet size of 200 or 300 μm was considered to be desirable. The objective behind the nozzle of U.S. Pat. No. 6,003,789 is to produce a spray of fine oil droplets which, according to the conventional view, would result in better contact between the coke particles and the oil droplets. A subsequent approach detailed in concept in “Injection of a Liquid Spray into a Fluidized Bed: Particle-Liquid Mixing and Impact on Fluid Coker Yields”, Ind. Eng. Chem. Res., 43 (18), 5663., House, P. et al., proposes that the initial contact and mixing between the liquid droplets and the hot coke particles should be enhanced, with less regard to the size of the liquid droplets in the spray.
A spray nozzle using a draft tube is proposed and described in U.S. Pat. No. 7,025,874 (Chan). This nozzle device functions by utilizing the momentum of the liquid jet issuing from the nozzle orifice to draw solids into the draft tube mixer and induce intense mixing of the solids and liquid in the mixer and by so doing, enhance the probability of individual droplets and particles coming into contact. As a result, more coke particles were likely to be thinly coated with oil, leading to improvement in liquid yield; the production of agglomerates would be curtailed, leading to a reduction in fouling and the reactor operating temperature could be reduced while still achieving high liquid product yield by reducing the mass transfer limitation on the liquid vaporization process. The actual assembly comprises an atomizing nozzle for producing the jet which extends through the side wall of the reactor and an open-ended draft tube type mixer positioned horizontally within the reactor and aligned with the nozzle so that the atomized jet from the nozzle will move through the tube and entrain a stream of coke particles and fluidizing gas into the tube where mixing of the coke and liquid droplets takes place. The draft tube preferably has a venturi section to promote a low pressure condition within the tube to assist induction of the coke particles and fluidizing gas. This device has not, however, been commercially successful due to concerns over fouling of the assembly in the fluidized bed.
The circular exit orifice on the nozzles shown, for example, in the Base and Chan patents, creates a cylindrical plume of liquid where the majority of the liquid is concentrated along the central jet axis, with limited ability of the entrained coke particles to penetrate to the central region of the jet; this plume has a minimum surface area to volume ratio and this creates a significant hindrance to the penetration of solid coke particles to the central core of the jet, leading to contact between the hot coke particles and the injected oil stream which is less than optimal.
US 2012/0063961 (Chan) describes an improved liquid feed nozzle useful in fluid coking units using heavy oil feeds such an oil sands bitumen which is fitted with a cloverleaf disperser at the outlet to provide a spray of liquid feed having an increased surface area relative to a cylindrical jet. The larger surface area of this plume increases solids entrainment into the jet and draws the liquid from the center of the jet to the lobes of the cloverleaf, improving the contact of liquid and solids in the fluidized bed.
While rectangular or slitted nozzles, as described, for example, in EP 454 416 (Steffens), U.S. Pat. No. 7,172,733 (Gauthier) and U.S. Pat. No. 5,794,857 (Chen), have been utilized to produce fan shaped sprays from the feed injectors for fluid catalytic cracking units, they are less desirable for use in fluid coking reactors because of the potential for plugging from excessive solids during process excursions. A certain minimum clearance is therefore required for the nozzle outlet, and a circular exit offers the greatest clearance. There is therefore a need for a nozzle assembly which is capable of improving the dispersion of the injected feed into the fluidized bed of coke particles in the fluid coking reactor.